1. Field of the Invention
The present invention relates to an optical amplification system using Raman amplification.
2. Description of the Related Arts
As a high-capacity technology, attention is being given to a wavelength division multiplexing (WDM: Wavelength Division Multiplexing) technology that allows a multiplicity of optical signals having different wavelengths to be transmitted through a single optical fiber. Moreover, in a WDM transmission system with the use of such a wavelength division multiplexing technology, a Raman amplification has been introducing to extend one span (the opposed node-to-node) interval longer.
Here, the Raman amplification means a technology to amplify signal lights, using an induction Raman diffusion phenomenon, one of non-linear effects within an optical fiber. When a signal light and an excitation light having shorter wavelength than that of the signal light for its specific Raman shift amount are inputted into an optical fiber, the signal light will be amplified within the optical fiber.
In other words, the excitation light having shorter wavelength than that of the signal light for its specific Raman shift amount allows a dipole to be generated within the optical fiber. And, optical amplification will be carried out, when a light whose wavelength is the same as that of the signal light is radiated, while returning to the normal order, after the energy is lost for the specific number of oscillations of the excitation light, with the signal light passing through.
FIGS. 1A to 1C show examples of a system with the use of such a Raman amplification. FIGS. 1A to 1C also show optical amplifiers (optical AMPS) 1 and 2 to be placed in the optical amplification system at each node (including the terminal station node and the repeater node) of one span-to-one span connected with a transmission path 3.
Generally, the optical amplifiers 1 and 2 are classified into a post amplifier that serves as a power amplifier when being placed at a transmitter of the terminal station node, and a preamplifier that amplifies weak signals when being placed at a receiver of the terminal station node. Moreover, when the optical amplifier is placed at the middle point of a circuit, in short, at a repeater node, it is classified as an in-line amplifier.
In a system shown in FIG. 1A, a Raman excitation light source (LS)4 is placed on the side of the optical amplifier 1, and the excitation light is sent out to an optical transmission path 3 in the same direction as the propagation direction of the main signal light. This mode is called a forward excitation. In a system shown in FIG. 1B, a Raman excitation light source 5 is placed on the side of the optical amplifier 2, and a Raman excitation light is sent out to the optical transmission path 3 in the reverse direction of the propagation direction of the main signal light. This mode is called a backward excitation. In addition, in a system shown in FIG. 1C, the Raman excitation light sources 4 and 5 are placed on the side of the optical amplifiers 1 or on the side of the amplifier 2, respectively, and the Raman excitation light is sent out to the optical transmission path 3 in the same or in the reverse direction of the propagation direction of the main signal light. This mode is called a two-way excitation.
Here, out of the three excitation modes, in the backward excitation (FIG. 1B) and the two-way excitation (FIG. 1C), the Raman excitation light is radiated in, in the reverse direction of the propagation direction of the main signal light. In such a case, at the upstream optical amplifier 1, the Raman excitation light will be radiated in from its output side.
Usually, on an optical amplifier, a laser ray having a power of 0 dBm through +20 dBm will be outputted to the optical transmission path 3 to be connected from the output area through a connector. Due to this reason, in consideration of the safety of a person who handles the device, the optical amplifier has a function (Laser Safety function) to reduce the output of the optical amplifier, detecting a Fresnel reflection at the location where the connector coming out of the output area of the optical amplifier.
However, as described above, in the backward excitation (FIG. 1B) and the two-way excitation (FIG. 1C), the Raman excitation light will be radiated in from the output side of the optical amplifier 1. Therefore, even if the connector is properly seated in place, due to the input of the Raman excitation light from the downstream, on the optical amplifier, a control function will be ON to reduce the output of the optical amplifier, misrecognizing the inputted light as a Fresnel reflection light to be generated if the connector is removed. In order to avoid such a failure, in normal operation, the Laser Safety function will be masked (stopped).
The masking treatment of the Laser Safety Function in such a normal operation will be controlled as follows, using the opposed circuit (transmission path).
FIG. 2 illustrates the masking treatment of the Laser Safety Function, and in this drawing, opposing optical amplification systems A and B are connected with the optical transmission path (OTP)3 having opposing transmission paths 30 and 31.
Assuming that the optical amplification systems A and B would be the end station nodes, optical amplifiers 1 and 10 correspond to the post amplifiers, and optical amplifiers 2 and 20 correspond to the preamplifiers. Also, if the optical amplification systems A and B would be the relay nodes, optical amplifiers 1, 2, 10 and 20 correspond to the in-line amplifiers.
The optical amplification systems A and B are connected with the transmission path 30 in the downward direction (direction from the optical amplifier 1 to the optical amplifier 2) and the transmission path 31 in the upward direction (direction from the optical amplifier 10 to the optical amplifier 20). The masking treatment of the Laser Safety Function will be controlled in the following procedure.
S1: If there is no trouble in the optical transmission path 3 (downward direction transmission path 30), the smooth passage of signals will be checked on the side of the downstream optical amplifier 2.
S2: The result of checking the smooth passage of signals will be transmitted to the optical amplifier 10 on the side of the transmission path 31 in the upward direction opposing to the optical amplification system B.
S3: In addition, from the optical amplifier 10, the check information will be sent to the optical amplifier 20 on the downstream side of the optical amplification system A through the upward direction transmission path 31, with the use of SV (monitor) signals.
S4: When the optical amplifier 20 for the optical amplification system A receives the check information, the information will be transmitted to the optical amplifier 1 on the side of the opposed circuit.
S5: By this procedure, to the optical amplifier 1, control is carried out to mask the Laser Safety function.
Moreover, on the systems as illustrated in FIG. 3, the following shows examinations on emergency procedures when the opposing transmission path 31 is in trouble.
S10: When the opposing transmission path 31 is OFF, S11: At the optical amplifier 20 for the optical amplification system A, OFF state of the SV (monitor) signal will be detected.
S12: The information that the SV (monitor) signal is in the OFF state will be transmitted to the side of the optical amplifier 1.
S13: Thus, the masking treatment of the Laser Safety Function of the optical amplifier 1 will be released, and the optical amplifier 1 will be controlled to continuously send out laser lights. However, in such a case, by releasing the masking treatment of the Laser Safety Function, the optical amplifier 1 will regard the Raman excitation light from the downstream side (side of the optical amplifier 2) as the reflection light to be generated when the connector is in the removed state, thereby causing the output to be lowered.
Here, as illustrated in FIG. 2, the masking treatment of the Laser Safety Function is complicated. Also in FIG. 3, if the opposing transmission path has any trouble, the optical amplifier 1 will be brought in the uncontrolled state because the information cannot be transmitted to the upstream.
From this reason, operators can be exposed to risks because signal lights of high output power will be continuously sent out due to the connector being removed from the amplifier. To prevent such risks, masking of the Laser Safety Function will be released. However, on the other hand, a problem can take place that a misrecognition of the input of the Raman excitation light as the Fresnel reflection light caused by removal of the connector could bring the output of the amplifier down to a safety light level.
Due to this, consequences of the troubled opposing transmission path could adversely affect the signal transmission of transmission paths having no trouble.
Therefore, for an optical transmission system without Raman excitation and an optical transmission system with Raman excitation, basic configuration of hardware should be changed. The change could lead up to an increase in cost for the resultant optical transmission systems. Moreover, this could be a significant bottleneck, in the event that an optical transmission system was made up without Raman excitation at the time of initial introduction, on the assumption that the optical transmission system would be sequentially extended, and a Raman excitation light source would be added as the increase in the number of wavelengths in the future.
It is therefore the object of the present invention to provide an optical amplification system that can eliminate such deficiencies.
In order to achieve the above object, according to a first aspect of the present invention there is provided an optical amplification system connected via a connector to an optical transmission path through which Raman excitation lights are sent out, the optical amplification system comprising an optical amplifier to amplify optical main signals; an optical receiver element to detect reflection lights from an end face of the connector, when the connector is disconnected; a circuit to reduction control the output power of the optical amplifier, based on the detection of the reflection light by the optical receiver element; and a blocking filter inserted between the optical amplifier and the connector, for blocking the Raman excitation lights.
In order to achieve the above object, according to a second aspect of the present invention there is provided an optical amplification system connected via a connector to an optical transmission path through which Raman excitation lights are sent out, the optical amplification system comprising an optical amplifier to amplify optical main signals; a Raman excitation light source to output Raman excitation lights; a wave synthesizer to synthesize optical main signals amplified by the optical amplifier, and the Raman excitation lights, for sending out to the transmission path; an optical receiver element to detect reflection lights from an end face of the connector, when the connector is disconnected; a circuit to reduction control the output power of the optical amplifier, based on the detection of reflection light by the optical receiver element; and a blocking filter inserted between the optical amplifier and the connector, for blocking the Raman excitation light.
In order to achieve the above object, according to a third aspect of the present invention there is provided an optical transmission system comprising a transmitter-side optical amplification system and a receiver-side optical amplification system which are connected through an optical transmission path through which a Raman excitation light is sent out, the transmitter-side optical amplification system including a connector connected to the optical transmission path; a pair of optical amplifiers to amplify wavelength multiplexed signals each having a different wavelength band; a coupler which synthesizes the outputs of the pair of optical amplifiers, for output to the optical transmission path through the connector; and a blocking filter placed between the coupler and the connector, for blocking wavelength bands of excitation lights for the wavelength multiplexed signals each having a different wavelength band.
In order to achieve the above object, according to a fourth aspect of the present invention there is provided an optical transmission system comprising a transmitter-side optical amplification system and a receiver-side optical amplification system which are connected through an optical transmission path through which are sent out Raman excitation lights which correspond to different wavelength bands, the transmitter-side optical amplification system including a connector connected to the optical transmission path; a pair of optical amplifiers to amplify wavelength multiplexed signals each having a different wavelength band; a blocking filter placed on the output side of one optical amplifier, of the pair of optical amplifiers, which amplifies wavelength multiplexed signals having a shorter wavelength band, of the different wavelength bands; and a coupler which synthesizes the output of the blocking filter and the output of the optical amplifier which amplifies the wavelength multiplexed signals having a longer wavelength band, of the different wavelength bands, for outputs to the optical transmission path through the connector, wherein the coupler further has a function to separate signals for each different wavelength band, the blocking filter having a band which blocks Raman excitation lights corresponding to the different wavelength bands.
In order to attain the above object, according to a fifth aspect of the present invention there is provided an optical relay connected to an optical transmission path through which Raman excitation lights are sent out, the optical relay comprising a connector connected to the optical transmission path; a pair of optical amplifiers for amplifying wavelength multiplexed signals having different wavelength bands; a coupler which synthesizes the outputs of the pair of optical amplifiers, for output to the optical transmission path through the connector; and a blocking filter placed between the coupler and the connector, for blocking wavelength bands of the excitation lights for the wavelength multiplexed signals having different wavelength bands.
In order to attain the above object, according to a sixth aspect of the present invention there is provided an optical relay connected to an optical transmission path through which Raman excitation lights are sent out, the optical relay comprising a connector connected to the optical transmission path; a pair of optical amplifiers for amplifying wavelength multiplexed signals having different wavelength bands; a blocking filter placed on the output side of one optical amplifier, of the pair of optical amplifiers, which amplifies wavelength multiplexed signals having a shorter wavelength band, of the different wavelength bands; and a coupler which synthesizes the output of the blocking filter and the output of the optical amplifier which amplifies the wavelength multiplexed signals having a longer wavelength band, of the different wavelength bands, for outputs to the optical transmission path through the connector, wherein the coupler further has a function to separate signals for each different wavelength band, the blocking filter having a band which blocks Raman excitation lights corresponding to the different wavelength bands.